As is known in the art, mechanical acceleration influences the frequency produced by oscillators. More particularly, mechanical vibration induces modulation of the RF frequency with associated unwanted phase noise. This is a well-known effect and long standing issue with RF oscillators. Various methods have been tried to reduce the acceleration sensitivity and/or reduce the vibration levels that are applied to the oscillator.
As is also known in the art, crystal oscillators operate at high frequencies (HF) through very high frequencies (VHF), while surface acoustic wave (SAW) oscillators operate at ultra high frequencies (UHF) and low microwave frequencies.
As is also known, radio frequency (RF) systems often require a UHF or microwave local oscillator (LO) signal, as derived from a SAW oscillator. Passive vibration isolators (e.g., mechanical vibration absorbers) are commonly used to reduce the effect of applied vibration on the SAW device. While these are effective at vibration frequencies well above the mechanical resonant frequency of the SAW oscillator, the vibration is amplified at the mechanical resonant frequency of the SAW device; these passive vibration isolators do not suppress vibration at frequencies below the mechanical resonant frequency of the SAW device. In addition, lowering the mechanical resonant frequency requires large sway space for the device.
Another technique used to reduce the effect of vibration on an oscillator is active vibration compensation. With crystal oscillators, accelerometers are used to sense vibration and in response to the sensed vibration modulate the oscillator frequency to reduce the effective oscillator acceleration sensitivity in the crystal oscillators. Active compensation is however only effective at low vibration frequencies due to bandwidths of the accelerometers, phase shifts in the compensation electronics, and mechanical cross-axis coupling. Furthermore, if a single crystal oscillator is passively isolated, interactions between phase shifts in the active compensation and the passive isolation may increase the levels of phase noise at certain vibration frequencies. U.S. Pat. No. 4,453,141 and later similar patents including U.S. Pat. No. 7,106,143, Bloch et al, issued Sep. 12, 2002, U.S. Pat. No. 5,250,871 inventors Driscoll et al. and U.S. Pat. No. 8,188,800 have described a method using two crystals which are physically oriented so that acceleration induced frequency shifts are opposing. lithe shifts are equal in magnitude and opposite in phase, they will cancel. Crystal acceleration sensitivity is not predictable, so this method requires screening large numbers of crystals. In addition, a mechanical gimbal is required. The mechanics introduce other problems including large size and/or degraded temperature stability. U.S. Pat. No. 4,575,690 describes a proposed improvement, but this method is also difficult to implement.
As is also known, UHF SAW oscillators are commonly locked to VHF or HF crystal oscillators to improve the frequency accuracy of the signal from the SAW oscillator. Crystal oscillators have superior frequency accuracy and stability as compared to SAW oscillators, (see “SAW Oscillators for Phase Locked Applications”, Joseph, T. R., Proceedings of 31st Annual Symposium on Frequency Control, 1977) while SAW oscillators have superior phase noise at higher offset frequencies than a multiplied crystal oscillator.